poison antidote.........

To take 'with a grain of salt' means to be skeptical or to question something that someone has told you. For example, if someone has a tendency to exaggerate, you'll want to take what they have said with a grain or pinch of salt.

The saying came from the old cure for poison - a pinch of salt. Pliny’s Naturalis Historia,77 A.D. translates thus: 

After the defeat of that mighty monarch, Mithridates, Gnaeus Pompeius found in his private cabinet a recipe for an antidote in his own handwriting; it was to the following effect: Take two dried walnuts, two figs, and twenty leaves of rue; pound them all together, with the addition of a grain of salt; if a person takes this mixture fasting, he will be proof against all poisons for that day.